Determine which device failed to open by looking for a FAULTED device
with an additional 'cannot open' message. If this device has been inadvertently
removed from the system, attach the device and bring it online with 'zpool
online':

# zpool online test c0t0d1

If the device is no longer available, the device can be replaced using
the 'zpool replace' command:

# zpool replace test c0t0d1 c0t0d2

If the device has been replaced by another disk in the same physical
slot, then the device can be replaced using a single argument to the 'zpool
replace' command:

# zpool replace test c0t0d1

Existing data will be resilvered to the new device. Once the
resilvering completes, the device will be removed from the pool.

For an exported pool

If this error is encountered during a 'zpool import', it means that one
of the devices is not attached to the system:

# zpool import
pool: test
id: 10121266328238932306
state: DEGRADED
status: One or more devices are missing from the system.
action: The pool can be imported despite missing or damaged devices. The
fault tolerance of the pool may be compromised if imported.
see: http://illumos.org/msg/ZFS-8000-2Q
config:
test DEGRADED
mirror DEGRADED
c0t0d0 ONLINE
c0t0d1 FAULTED cannot open

Unlike when the pool is active on the system, the device cannot be
replaced while the pool is exported. If the device can be attached to the
system, attach the device and run 'zpool import' again.

Alternatively, the pool can be imported as-is, though it will be placed
in the DEGRADED state due to a missing device. The device will be marked as
UNAVAIL. Once the pool has been imported, the missing device can be replaced as
described above.